Apparatus for accelerating electrically charged particles



R. WIDERUE Dec. 30, 1952 APPARATUS FOR ACCELERATING ELECTRICALLY CHARGEDPARTICLES 2 SHEETS-SHEET 1 Filed March 8, 1951 YWPONPW ATTORNEYS Dec.30, 1952 R. WIDERGE Filed March 8, 1951 2 SHEETSSHEET 2 INVENTOR BY Wsvzl mg/fimkw ATTORNEYS Patented Dec. 30, 1952 APPARATUS FORACCELERATING ELEC- TRICALLY CHARGED PARTICLES Rolf Wideriie, Ennetbaden,Switzerland, assignor to Aktiengesellschaft Brown, Boveri & Cie, Baden,Switzerland, a joint-stock company Application March 8, 1951, Serial No.214,471

In Switzerland December 1, 1949 This invention relates to acceleratorsfor electrically charged particles such as electrons wherein theparticles are forced to'travel an orbital path of substantially constantradius while at the same time being constantly accelerated along thepath, and is a continuation-in-part of my application Serial No. 198,547filed December 1, 1950, and now abandoned.

One type of accelerator, known generally as a. betatron or raytransformer," to which the present invention can be applied, iscomprised of an evacuated annular tube into which electrons areintroduced from an electron emissive' cathode, and a magnetic systemwhich produces a magnetic field varying with time having a spatialdistribution such that the injected electrons are accelerated along asubstantially circular orbit within the tube by magnetic inductioneffects. The magnetic field divides into two components. duction fieldis responsible for acceleration of the electrons around the orbit, andthe other component known as the control field produces an increasingcentripetal force upon the electrons which is designed to match theincreasing centrifugal forces of the electrons as the latter increasesin velocity thus maintaining the electrons on a circular path ofsubstantially constant radius. The electrons reach enormously highenergy levels as a result of their acceleration and at the end of theacceleration phase can be diverted from the orbit and caused to impingeupon an anode to produce X-rays or used for other purposes.

The betatron type of accelerator usually incorporates a stabilizingdevice for restoring to the prescribed circular orbit those electronswhich deviate from it. Such stabilization can be achieved for example byestablishing a gradient in the control field such that the strength offield decreases outwardly with increasing radius of the electron path,and a full discussion of the apertaim'ng theory can be found in U. S.Patent No. 2,103,303, issued December 28,1937, to Max Steenbeck. Thestabilization attributable to the decrease in field strength iseffective only within a limited vicinity of the precalculated circularorbit for electron travel, known generally as the circle of equilibrium,but within the effective area provides both radial and axial stabilizingforce components for any electrons which leave the precalculated orbit.In the plane of the equilibrium circle, the stabilizing force has twomaxima, which lie on circles located radially inward and outwardrespectively from the circle of, equilibrium.

One component known as the in- 9 Claims. (Cl. 313-62) Heretofore, it hasbeen the general practice to locate the anode and its necessarysupporting structure, against which anode the stream of electrons iscaused to impinge for the production of hard X-rays, outside of theefiective area of stabilization, that is, outside of the two limitcircles of maximum stabilizing forces, since it was thought that toomany electrons would be lost from the stream by impingement were anobstacle to be placed anywhere in the same gene eral area whereelectrons might be expected to travel during their acceleration phase.

Experience, however, has shown that the yield of hard X-rays for examplefrom a betatron varies somewhat inversely as the radial distance whichthe electrons must travel from the circle of equilibrium to reach theanode. One possible explanation for this undesirable effect is that inthe vicinity of the maximals of the stabilization forces, localdisturbing fields enter into the control field space from without withthe result that the inner and outer circles of maximum stabilizing forcewhich define the efiective area of electron stabilization lie closer tothe circle of equilibrium than is calculated.

The general objective of this invention is' to provide an improvedarrangement for the anode within the annular tube that will assure amaximum yield of accelerated electrons available for impingement againstthe anode and such objective is achieved by employing a support there--for extremely thin in the dimension perpen dicular to the plane of thecircle of equilibrium and which locates the anode in the plane of theequilibrium circle at a point between the limits of one half and fourfifths of the distance to the equilibrium circle from either the inneror outer limit circles of maximum stabilizing force.

The foregoing as well as other objects .and advantages inherent in theinvention will be-. come more apparent from the following descriptionand accompanying drawings illustrating two practical embodiments of theinvention.

In the drawings, 1

Fig. l is a view in central vertical section of an accelerator forelectrically charged particles known as a betatron, wherein electronsare accelerated by 'magnetic inductive efiects exclusively, to which theinvention has been applied.

Fig. 2 is a view partly in horizontal section and partly in plan of theaccelerator shown in Fig. 1;

Fig. 3 is a curve used in explaining the invention;

Figs. 4 and 5 are views in top plan and side elevation of the anode andits supporting ture drawn to-an enlarged scale; and

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Figs. 6 and 7 are views similar to Figs. 4 and 5 illustrating a somewhatmodified supporting structure for the anode.

Referring now to Figs. 1-5 in particular, the electron inductionaccelerator, which is symmetrical about a vertical axis a-a, iscomprised of a magnetic structure l made up from steel laminations ofappropriate contour and includes a pair of confronting cylindricalinduction poles ll|l separated by air gap' 12 located eoncen' tricallyalong axis a-a, and a pair of confronting annular control poles l3 l 3'separated by air gap I4, the control poles being locatedradially outwardfrom the induction poles and also concentric with axis a/a. Theconfronting faces of the control poles are preferably tapered in suchmanner that the air gap I 4'increases' as measured in the radiallyoutward direction from axis a a, thus effecting a corresponding decreasein strength of the control field. I

Yokernembers [5 complete the magnetic circuit for'the time variedmagnetic field set up in the control and induction poles. A winding preferably split into" two annular coil sectionslfi, l6 surrounding theupper and lower control poles 13 3" respectively and wound in the samedirecutn are connected in series for energizanon from a source ofalternating current of suitable frequency such as for example 100cycles/see,

such source being indicated simply by terminals H. An annular evacuatedtube It, preferably of glass, is disposed symmetrically in the air gap[4 between the control poles l3l3'. The timevaried current in energizingcoils lfi-IS' produces the time-varied magnetic field previouslymentioned. The induction component of this field responsible forelectron acceleration passes through induction poles l [-4 l acrossair'gap I2, and the control component responsible for maintaining theelectrons on a circular path of cons'tant ra'dius' passes through polesl3l3 across air gap ['4. In Figs. 1 and 2, the circular path or circleof "equilibrium is designated k and its radius hidicated by Re, theplane containing the equilibrium circle Ic being perpendicular to axisaa.

Aspr'eviously explained, all of the electrons do not always adherestrictly to the path It but rather deviate from the same. In order tostabilize the electron path, the control field is made to decrease inthe" radial direction by tap'ering the faces of the control polesi3''l3'. stabilization force P attributable to this radial gradient in"the control field has a characteristic as shown by the curve in Fig. 3which shows the course of the force in a plane perpendicularto theequilibrium circle is. At the circle of equilibrium 7e, radius R0, forceP, is zero. In a direction radially outward from circle ic, thestabilizing force Prisesluniformly in one direction reaching a'maxirnaltime an outer limit circle m' of radius Re. In the direction radiallyinward from circle e, thestabilizing force P rises uniformly in theother direction reaching a maximal along' an in-' ner limit circle n ofradius R1. The forceP drops quickly after reaching the maximal attheinnr'an'd outer limit circles. For locaticnsRd and Rathereapplies thecondition R+B="max or Bibeing the strength of the control field.

The cathode 2U constituting the source of the The 4 ing limit circle n.In an experimental model which has proven very satisfactory, a very thinplate 22 whose dimension in a direction perpendicular to the plane ofthe circle of equilibrium is at most one-tenth of a millimeter projectslaterally from housing 2| in a radially outward direction for aboutmillimeters and at the outer end thereof the plate 22 supports the anode23 which is spaced from the equilibrium circ'lelc by at least one halfand by at most four fifths of the radial distance d to the equilibriumcircle lc fromthe inner stabilizing limit circle 11. The plate 22 whichmay be made from platinum or a platinum-iridium alloy preferably iswider at its point of attachment to housing 2| than at the end carryinganode 23 for easier attachinent to the housing and also in order toplace its mechanical inherent vibration frequency as high as possibleabove the frequency of the current used in energizing the coils I-6-|6.In the plane perpendicular to the plane of the equilibrium circle is,the support plate 22 therefore presents only a very small areaandconstitutes, during a small, initial fraction of the acceleration periodduring which the electrons still deviate appreciably from theequilibrium circle, only a very small obstacle to elec-' tron travelwithin the total area of stabilization. Consequently the number ofelectrons lost from the stream because of impingement with support 22 ispractically negligible; During the subsequent longer remainder of theperiod in which the electrons are accelerated the deviation of theelectrons from the equilibrium circle becomes much smaller and none ofthe electrons strike the support 22 or anode 23. 7 I

Since the electrons are scattered as they enter the anode 23, it isadvisable to give the anode 23 which preferably consists of platinum, asomewhat larger dimension in the direction perpendicular to the plane ofthe equilibrium circle is than is given the support plate 22-, as seenmore clearly in the enlarged Figs. 4 and-5, so that the face of theanode ofiers to the incoming accelerated beam of electrons an area ofabout 0.25, or 0.5, or at most LOsquare millimeter; This area is muchsmaller than the area of plate 22 in the same plane, the length of theplate being about fifteen millimeters. The diniension of the anode 23and plate 22 in the direction parallel with the plane of the equilibriumcircle I k in the experimental model was about 2 millimeters, and thecathode was located approximately the same distance from the inner limitcircle n as the anode 23.- I-he necessary control circuits by which thecathode 20 is energized periodically in timed relation with thevariation in the magnetic field to inject a stream of electrons into thetube at the beginning of each cycle or half cycle of the alteratingmagnetic field have not been illustrated since; various arrangements arealready well known and moreover are not considered essen anunderstanding of this invention which concerns only the placement of theanode and cathode elements within the tube, and the support for theanode.

In the betatron type" of accelerator, meansmiist be provided for streamfrom the orbit Ic acceleration period so as against the anode 23.

accomplishing this result between the induction and control fieldcomponents such as by eifecting a partialn'iagnetic diverting theelectron towards the end of the to cause it toimpinge One practical wayof is to change the-ratio saturation in the magnetic circuit. In theillustrated embodiment of the invention wherein the anode is disposedradially inward of the orbit is, the induction poles ll-l I would thuseach include a portion Ila, Il'a of restricted area so as to effect apartial magnetic saturation in these poles as the induction fieldreaches a predetermined magnitude. Thereafter the centripetal forcesattributable to the control field would exceed the centrifugal forces ofthe electrons thus driving the electrons radially inward from the orbitk.

If desired, the cathode and its housing may be located radially outwardfrom the outer stabilizing limit circle m in which case the anode wouldbe disposed at a corresponding position between the outer limit circle mand the equilibrium orbit it. Such an arrangement would of courserequire the electron stream to be diverted radially outward from theorbit is for impingement against the anodes and could be effected byproducing a partial magnetic saturation effect in the control poles.

In lieu of the support plate 22, Figs. 6 and 7 show an arrangementwherein the anode support is constituted by a U-shaped stirrup 28 whosetwo ends are secured to the cathode housing 21. The anode 28 is securedat the outer end of the stirrup, and like plate 22, the dimension of thestirrup in the direction perpendicular to the plane of the equilibriumcircle is would be very small, that is, of the order of .1 millimeter orless so as to present as small an obstacle as possible to the run of theelectrons around the orbit.

In conclusion, while I have described and illustrated my invention asapplied to a betatron wherein electron acceleration is produced whollyby the principles of magnetic induction, it can be applied equally aswell to other types of accelerators such as the synchrotron whichemploys a magnetic structure similar to that used in the betatron butuses the magnetic field produced by it principally for guiding theelectron stream in the circular path. The initial acceleration of theelectron stream can be effected by magnetic induction but thereafterfurther energy is added to the stream by causing it to traverse aresonant cavity containing a gap to which a high frequency potential isapplied such as described and shown in U. S. Patent No. 2,485,409,issued October 18, 1949, to H. C. Pollock et al.

Moreover while I have shown the anode as being carried by a supportsecured to the cathode housing, the support could be secured in someother manner within the tube without departing from the spirit and scopeof the invention as defined in the appended claim.

I claim:

1. In a device for accelerating charged particles comprising a magneticstructure including a pair of axially aligned circular control poles thefaces of which are arranged in confronting relation to establish a gaptherebetween, an annular tube disposed in the gap between said controlpoles providing therein an orbital path for acceleration of theparticles, means for producing a time varied magnetic field across saidgap between said control poles to confine the particles to asubstantially circular path coincident with a circle of equilibriumduring the acceleration phase, said pole faces being divergent in aradially outward direction whereby the strength of said field decreasesin a direction radially outward from the axis of said control poles toestablish a stabilizing force for the particles having in the plane ofsaid equilibrium circle two maximals lying respectively on limit circleslocated radially inward and outward of said equilibrium circle, an anodein said tube disposed in the plane of said equilibrium circle and spacedfrom the latter by at least half but not exceeding four fifths thedistance to the said equilibrium circle from one of said limit circles,and a support within said tube for said anode, said support beingrelatively thin as measured in a direction perpendicular to saidequilibrium circle.

2. A device for accelerating charged particles as defined in claim 1wherein said anode is constituted by a body of platinum having one facedisposed perpendicular to the plane of said equilibrium circle having anarea not exceeding one square millimeter.

3. A device for accelerating charged particles as defined in claim 1wherein the dimension of said anode support in a direction perpendicularto said equilibrium circle is at most one-tenth millimeter.

4. A device for accelerating charged particles 5. A device foraccelerating charged particles as defined in claim 1 wherein the supportfor said anode consists of platinum.

6. A device for accelerating charged particles as defined in claim 1 andwhich further includes a cathode housing disposed within said tube atthe side of the said one limit circle away from said anode, and acathode within said housing for producing the said charged particles,said cathode being located substantially the same distance from said onelimit circle as is said anode. and said anode support being attached tosaid cathode housing at the end opposite that at which said anode isattached.

7. A device for accelerating charged particles as defined in claim 6wherein the support for said anode at the end attached to said cathodehousing has in the plane of the equilibrium circle a larger dimensionthan the end supporting said anode.

8. A device for accelerating charged particles as defined in claim 7wherein the support for said anode is constituted by a fiat plate.

9. A device for accelerating charged par- I ticles as defined in claim 1and which further includes a cathode housing disposed within said tubeat the side of the said one limit circle away from said anode, and acathode within said housing for producing the said charged particles,said cathode being located substantially the same distance from said onelimit circle as is said anode, and said anode support is constituted bya U- shaped stirrup attached by the ends thereof to said cathodehousing.

ROLF WIDERE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,335,014 Kerst Nov. 23, 19432,484,549 Blewett Oct. 11, 1949 2,550,212 Wideroe Apr. 24, 19512,553,312 Gurewitsch May 15, 1951 2,558,597 Westendorp June 26, 1951

